Free-machining steel, articles thereof and method of making

ABSTRACT

Improved free-machining, resulfurized and rephosphorized carbon and low alloy steels, especially gun barrel steels, method of making the same, and articles thereof containing 0.015 to 0.15 weight percent zirconium for improved chemical and structural homogeneity, enhanced hot workability and hot-rolled surface quality, increased strength, and superior hardness uniformity.

United States Patent John G. Cutton Board Township, Mahoning County; George A. Welsch, Jr., Youngstown, both of Ohio July 9, 1969 Jan. 1 l, 1972 United States Steel Corporation Inventors Appi. No. Filed Patented Assignee FREE-MACHIN IN G STEEL, ARTICLES THEREOF AND METHOD OF MAKING 6 Claims, No Drawings US. Cl 75/123 D, 75/123 G, 75/123 H, 75/123 L, 75/126 F, 75/126 Int. Cl ..C22c 39/26, C22c 39/44, C220 39/54 Field of Search 75/123,

[ 56] References Cited UNITED STATES PATENTS 2,157,673 5/1939 Ridgely 71 2,182,758 12/1939 Harder 71 3,169,857 2/1965 Rathke 75 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Joseph E. Legru Attorney-John R. Pegan ABSTRACT: Improved free-machining, resulfurized rephosphorized carbon and low alloy steels, especially barrel steels, method of making the same, and articles the containing 0.015 to 0.15 weight percent zirconium for proved chemical and structural homogeneity, enhanced workability and hot-rolled surface quality, increased strer and superior hardness uniformity.

BACKGROUND OF THE INVENTION TABLE I carbon 0.08 to 0.55%

manganese 0.30 to 1.65% silicon up to 0.30% sulfur 0.08 to 0.35% phosphorus up to 0.12%

(over 0.04 for rephos horized steels) balance, except for incidental impurities.

iron

Resulfurized and rephosphorized alloy steels are also known to the art, for example, low alloy steels which, while conforming to the above table I compositional range, are strengthened or otherwise modified by the incorporation of additional alloying elements such as chromium, nickel, molybdenum, copper, vanadium, titanium or boron. lllustratively, such steels may contain chromium up to about 1.5 percent, nickel up to about 3.8 percent, molybdenum up to about 0.60 percent, copper up to 1.00 percent, vanadium up to about 0.30 percent, columbium up to about 0.05 percent, titanium up to about 0.03 percent, and boron up to about 0.006 percent. Reference herein and in the appended claims to low alloy" steels is intended to include steels within the table I compositional range with one or more additional alloying elements as aforesaid up to a total of such alloying element additions of about 4 percent. Like the mentioned carbon steels, such low alloy steels may also contain lead, in amount of about 0.15 to 0.35 percent for machinability enhancement. Exemplary of the resulfurized low alloy steels of the prior art are steels having compositions within the range: 0.47 to 0.55 percent carbon, 0.95 to 1.30 percent manganese, 0.60 to 0.90 percent chromium, 0.06 to 0.10 percent sulfur, and balance substantially iron.

The large percentages of sulfur and/or phosphorus in prior art free-machining steels tend to the formation of relatively large sulfide inclusions, or segregates high in sulfur and phosphorus contents, whereby the full machinability benefits of these elements are not realized and internal soundness and microstructural homogeneity of ingots of such steels and products made therefrom are adversely affected. Moreover, such high-sulfur, high-phosphorus prior art steels are prone to hot shortness, with resultant difficulties in hot working, and hot-rolled products thereof commonly present surfaces of low quality with many seams resulting in poor mill yields.

Attempts have been made in the past to correct these deficiencies of such free-machining steels, for example, by addition of alloying elements, such as boron (Murphy US. Pat. No. 2,388214) to inhibit sulfur segregation. Boron, however, even is small amounts is very potent in its effect on hardenability of steels, and even slight inhomogeneities in boron-containing steels intended for critical machining applications sometimes result in large variations in hardness and hence in variable machining characteristics. Further, boron is rendered largely ineffective if added to other than fully aluminum deoxidized steels.

SUMMARY OF THE INVENTION The present invention provides a method for the increa; mill yield in the production of new and improved resulfut and rephosphorized free-machining steels which ex] enhanced hot rollability, improved hot-rolled surface qu and improved ingot soundness and structure, inclu decreased segregation of constituent elements, especially fur, phosphorus and carbon. The new steels also show creased yield and tensile strengths and more uniform hard as compared to prior art steels. In a preferred embodim these advances are achieved by the addition of at least at 0.015 percent, preferably 0.015 to 0.15 percent, especi 0.015 to 0.05 percent, zirconium to a base composition c prising, by weight percent:

TABLE II carbon 0.30 to 0.45%

manganese 0.70 to 1.0% silicon 0.10% maximum sulfur 0.08 to 0.15% phosphorus 0.07 to 0.12%

iron balance, except for incidental impurities.

DESCRIPTION OF THE INVENTION Zirconium is known is the metallurgical arts as a useful dition to special purpose steels, for example, deep-draw steels and low alloy, high-strength steels, for its deoxidiz and grain refining effects. Selmi et al. US. Pat. No. 22,( teaches the addition of 0. 10 to 0.15 percent zirconium to l alloy steels of good deep-drawing properties, high fatigue sistance and freedom from strain and quench aging. Bidl U.S. Pat. No. 2,840,872 discloses high-strength, low all steels containing 0.03 to 0.11 percent zirconium as a de( idizer added to the mold or ladle in the form of a zirconiu and aluminum-containing material. Such known metallurgii effects of zirconium as an alloying additive to steels is recc nized in the Metals Handbook, vol. 1, page 90, 8th editir 1961, published by the American Society for Metals. Crafts, U.S. Pat. No. 2,280,283 discloses the addition of zirconium titanium, cerium, hafnium, thorium, vanadium, columbiul tantalum or uranium, in amount of 0.03 to 1.0 percent, further enhance deep hardenability conferred by one or combination of the elements boron, silicon, molybdenui tungsten, chromium or nickel. Crafts, too, recognizes t1 grain-refining efiect of the several elements of the above-me tioned zirconium-containing group of elements, but deriv the additional deep-hardenability effect from an additir larger than that required for grain refining.

We have now found that zirconium, when added in certa restricted amounts to resulfurized and rephosphorized fre machining carbon and low alloy steels, contributes to SLK steels certain new and beneficial properties and effects aboi and beyond those heretofore known to or practiced by tl metallurgical arts.

Many free-machining steels containing high percentages r sulfur and phosphorus are difficult to hot roll, and the ho rolled surface is of poor quality. Exemplary of such steels AISI grade 1140, having a composition essentially as given i table 11 above. Such steels, furthermore, when cast, commonl are productive of ingots having widely varying chemical cor stitutions, e.g., from center to surface of the ingot, togethr with a significant microstructural heterogeneity. The latte characteristic is particularly evidenced by the presence in it gots of such steels of large sulfide inclusions and sulfuran phosphorus-containing segregates. Still further, ingots of suc steels, cast by usual steel mill practices, exhibit a high degre of subsurface porosity. These ingot defects commonly con tribute to high product loss and low semifinished mill yield: e.g., on the order of 60 percent.

lirectly, and to another part of such heat a mold addition of zirconium was made, in the form of pounds of an alloy (1 round of zirconium per ton of steel) which analyzed to the folowing approximate composition, by weight percent:

TABLE III Macroexamination was made of the first (top) three and the last (bottom) billet produced from each of the foregoing ingots 7 and 8, from which it was observed that the zirconiumcontaining billets showed little or no visible segregation or subsurface porosity, whereas the zirconium-free billets showed heavy center segregation and extensive subsurface porosity. Despite the inherent deoxidizing capacity of zirconium, the relatively small amount added was such that the steel behaved as a semikilled steel, and n0 pipe was observed on the top billets of the No. 7 ingot.

The billets above mentioned were further reduced by hot rolling to 1-inch rounds. To this point, total mill yields were:

silicon 51.17%

zirconium 39.23% TABLE iron 9.10% l 5 carbon 0.44%

Percent Mill Yield he ingots were 21x23 inches in cross section and weighed zirconiumwmaining Steel 65% Jout 7,900 pounds each. The ladle analyses of these steels Zirconium-free steel 64.87% ere as given in the following table:

TABLE IV m Igngot Composition, weight percent aat Number ber Type Mn Si S P Cr A1 Zr Fe D300 8 at"--- .89 .07 .12 .09 .18 .002 .003 Balance! X43114 7 Zr-eontainln .93 .16 .12 .10 .13 .002 .027 D0.

Except for usual steel-making impurities.

Such ingots were hot-rolled to Z /z-inch-square billets, durg which it was observed that the KX-Ol l4 hot-rolled oduct had a good, smooth surface, as contrasted with the egular" 41D300 product, the surface of which was highly amed.

zirconium-treated, from regular parent heat got No. 7 (KX-0114 Composition, weight percent M Ingot uple Numbers location 0 Si 8 P Zr Top, No. 41 13 14 11 054 l. .40 12 .12 l1 034 Top, No .43 .12 .16 .14 .034 2. -10 12 12 11. 038 Top, No 42 11 .15 13 042 11 .0 6 0 -"i as .10 10 .09 047 Average percent Zr 0. 044

Ingot N o. 8 (regular heat 41D300) center op, N o. 59 10 28 22 midway 43 l0 13 13 :enter ..}Top, N o. 47 09 22 19 midway 37 10 11 11 None :enter }Top, N o. 41 08 14 13 nldzvay. 3. 23g 1 cell an. midway- 30 06 08 09 rom the chemical segregation data of table V, it is seen the zirconium-treated product was significantly less 'egated, as regards carbon, sulfur and phosphorus, than the untreated product, thus:

TABLE VI Ave. difference (wt.% X 10) between center and midway analysis (from table V) 4O Zirconium-free steel Micro examination of the respective 1-inch round samples showed a slight grain refinement by the McQuaid Ehn grain size test,

TABLE VIII Grain Size Zirconium-containing steel Duplex: 6 to 8; 10% 2-4 2-4 (ave. 3).

and a more evenly distributed microstructure, the zirconiumcontaining product being essentially free of large pearlite 45 areas which were prominent in the nontreated steel.

Strength and ductility of duplicate test specimens of the foregoing table IV comparison steels were asgiven in table 1X:

Thus, the zirconium-containing steel exhibited a significant strength increase while retaining excellent ductility.

Hardness tests were made across the cross section of the 1- inch round specimens, with resultsas set forth in table X:

TABLE X.-HARDNESS, RB

Distance from surface, inches Specimen Steel Number 0 0.15 0.30 0.50 0.30 0.15 0

1 88 89 9o 90 9o 90 89 zt'cmtamng 2 as s7 s7 s1 s7 81 as Z t 1 as s4 s2 s2 s4 86 88 2 86 s5 83 82.6 86 81 as As seen in table X, the specimens from the zirconium-containing steel, KX-Ol 14, showed more uniform hardness from surface to center than did those of the zirconium-free steel, 4 l D300. Such superior hardness uniformity, together with the enhanced microstructural homogeneity exhibited by such new steels, is especially desirable in demanding machining applications such as long bore reaming, etc., wherein accurate cutting tool position is critical and is facilitated by uniformity of the steel being machined.

Further tests were carried out wherein varying amounts of zirconium, both in the form of zirconium-silicon alloy and zirconium metal, were added to gun barrel quality carbon steel (with added chromium).

TABLE XI I t Composition, weight percent (ladle analysis) rigo Steel Number Mn Si S P Cr Zr Fe 52D524- 2 .35 .94 .05 12 .09 .15 .003 Balance KX-OIM- 5 35 95 05 13 09 15 011 D0. TEX-0135.-. 6 35 1. 03 05 14 t 014 Do. KX-013G. 7 35 1. 01 05 14 10 15 030 D0. KX0137 8 35 98 05 14 10 15 026 D0.

1 Except for normal steelmaking impurities.

As in the preceding test, the 2 l X23 inch cross section ingots were hot rolled to 2' -inch square billets, and were made of the billets to determine chemic center and midway positions. Representative results are given in table Xll:

-inch drillings al analyses at TABLE XIII .2% oflset Percent Percent .53., U.T.S., elon ation Steel Zr p.s.i. p.s.i. in 2 riches 5 88,530 27.0 89,870 27.0 97 860 24.0 25.0

from which it will be seen that, in the range 0.014 to 0 percent zirconium, the steels investigated showed enhai yield and tensile strengths with excellent retained ductili 1 5 compared to the zirconium-free steels.

Macroexamination of the top three and the bottom billets fromeach of the table XI ingots revealed the prest of extensive visible center segregation and subsurface port in the untreated steel billets (from lngot No. 2). Si

20 segregation and porosity was observed in the billets from li No. 5 (0.01 1 percent Zr), while the billets from the highe ingots, lngot No. 6 (0.014 percent Zr), No. 7 (0.030 pert Zr) and No. 8 (0.026 percent Zr) were found to be essenti free of visible segregation and subsurface porosity.

We have also found the above-described free-machii steels containing zirconium to be of greatly improved hot v kability as compared to the same steels without the zircon addition. Thus, usual mill practice with the 21x23 inch in; above described is to (I) reduce the ingot, from an ini ingot hot-rolling temperature, e.g., 2,200 F to an in mediate-sized article, e.g., to an 18%X20 inch cross secti (2) reheat to the hot rolling temperature, and (3) thereai roll the article to the form of a 2- to 3-inch square-section T L let. Attem ted direct rollin of the above-described zircc AB E XII f P b i g l 7 umree gun arre steels from the origina ingot to 2/a-ir St 1 2 I as 5 D524 (lngot N0 2 Zr free) billet form resulted in disintegration of such articles dur Composition, weight percent ro]1in Sample C 51 s P We have found, however, that the same steels, with the i center 41 05 14 i m 001 d tion of at least about 0.015 percent zirconium, can be 1'0! 2-1 midway A4 '11 without reheating, from ingots of, e.g., l9X22 inch cross SI gjjggggg" 2? -8 tion, directly to 2 /a-inch billets without any difficulty wh 2-3-center: I ever and, further, that such latter products have a higl 2-3-midwa 38 06 13 10 003 2 B cenmt 1 31 07 u 08 002 quality hot rolled surface, free of seams, than do the prior 2-B-midway .33 .06 11 09 ,003 4 5 steels even when the latter are sub ected to intermediate he: .sAtVBB%3X d i{1)T1e3r5en(cIa (10 0 4. 5 5 2. 0 i

ee 11 0t No. 014 Z from a t 2D52A fi-i-cer ter ise .08 i 22 F We believe, but without being bound by this theory, th

6- ml way 740 .08 .16 .13 .015

6 2 cemer 46 09 20 15 013 such improved hot workability of our new steels s due, at le.

, '12 in part, to the diminution or substantial elimination in the n:

:3? :82 :3}? 0 steels of large sulfide inclusions and sulfurand phosphort .35 .08 .12 .11 017 containing segregations. Thus, we have observed that, qui

' 017 unexpectedly, the addition of at least about 0.015 percent zi Average percent Zr 0. 148 conium to resulfurized and/or rephosphorized free-machinii Average difference 5 3 5 1'75 steels, such as the aforementioned gun barrel steel, results large sulfides being broken up and dispersed substantial 1 Bottom billet.

It is seen from table Xll that as little as about 0.015 percent zirconium is effective in these free-machining steels to substantially diminish segregation of carbon, sulfur and phosphorus. Comparison with the data of table V, however, will show that somewhat greater amounts of zirconium, i.e. 0.044 percent or higher, provide a still greater effect on segregation elimination.

A semifinished (ingot to billet) mill was realized in the production of the foregoing billet products of the zirconium-treated steel, as compared to a yield of 60.9 percent for the untreated steel products. Increased yield resulted principally from decreased center segregation in the zirconium-containing steel.

These zirconium-containing steels exhibit machinabilities equal or superior to those of the zirconium-free machining steels.

Physical properties of re hot rolling to 15/ 16-inch rounds, are given in table XIII:

yield of 81.6 percent presentative table Xl steels, after uniformly throughout the steel article. This ability of such zi conium additions to accomplish a dispersion of sulfide incli sions without any pronounced change in the morphology of ii clusions, we believe to have improved both the hot-rollir 60 characteristics of the steel ingots and the internal soundne:

and homogeneity (as shown by macro etch tests).

Further, phosphorus has a similar tendency to segregai with the sulfur in the steel. With the sulfur of sulfide inclusior dispersed, the phosphorus has also been dispersed with a cor comitant improvement in properties of the steel.

As above described and shown, the aforementioned benef cial effects of zirconium are obtained by the addition of fret about 0.015 to about 0.05 percent of that element. About 0.0 percent zirconium is a preferred upper limit of that elemer whereby the most significant benefits of the invention are 0!: tained at lowest cost. Larger percentages of zirconium may b added within the limits prescribed by economics and/or in teraction of these and other effects of zirconium upon othe desired steel properties. Broadly, we contemplate the additioi of zirconium up to about 0.15 percent for the purpose 0 achieving the desired results herein taught. Greater amounts may be used to achieve supplemental results as may be desired.

The use of such zirconium additions is contemplated for any free-machining carbon or low alloy steel wherein the aforesaid newly discovered beneficial effects of such addition may be utilized with advantage. For example, free-machining steels :ontaining segregation-prone elements as selenium or tellurirm in supplement to or in place of sulfur are included within he scope of the invention.

Our steels preferably contain carbon in the range of about ).l to 0.60 percent together with silicon up to about 0.30 vercent, preferably up to about 0.10 to 0.15 percent silicon. \n adverse effect of silicon upon machinability becomes uronounced in steels having a carbon content below about '08 to 0.10 percent. Therefore, at the lower end of the can on range, especially below about 0.14 percent carbon, silicon hould be held at the low end of its range. in the case of such teels, the zirconium is preferably added as elemental zirconim metal, to avoid introduction of silicon, as in iron-siliconirconium alloys. Carbon is preferably held to 0.55 or 0.60 ercent maximum, especially 0.45 to 0.55 percent, although )ecial applications, requiring higher carbon up to 1.0 or 1.5 ercent, may also benefit from the principles of the invention.

Broadly, manganese may be present in out improved freeachining steels in amount from 0.30 to L65 percent. Manmese contents above the minimum, e.g., at least about 0.70 :rcent, are preferred however, especially when sulfur is in e high end of its range.

In view of the ready combination of zirconium with oxygen molten steel, it is highly desirable that the steels be at least lrtially deoxidized or'killed prior to or along with the addim of zirconium. We have found zirconium utilization eifisncies of 50 to 80 percent to be readily achievable in the oduction of semikilled steels such as the gun barrel steels exnplified above, wherein the zirconium additions were made, ter silicon blocking, to either ladle or mold.

It is to be understood that the foregoing description and ecific examples are illustrative of the principles of the invenm, and that various modifications and additions rnay be made thereto by those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.

We claim:

1. An improved free-machining semikilled gun barrel steel consisting essentially of, by weight percent,

carbon 0.30 to 0.45% manganese 0.70 to L0; silicon 0.10% maximum sulfur 0.08 to 0.l5% phosphorus 0.07 to 0.]2k zirconium 0.015 to 0.07

iron balaricetexcepl for incidental impurities,

tains up to about 0.20 percent chromium.

4. A method of producing free-machining resulfurlzed and rephosphorized carbon steel articles of improved chemical and microstructural homogeneity and enhanced hot workability, comprising partially killing a molten steel containing carbon in an amount from 0.30 weight percent to 0.45 weight percent, sulfur in an amount from 0.08 weight percent to about 0.15 weight percent and phosphorus in an amount from 0.07 weight percent to about 0.12 weight percent, and adding thereto a zirconium-containing material in amount sufficient to provide the steel with a zirconium content of at least about 0.015 weight percent.

5. A method in accordance with claim 4, wherein the steel contains from 0.70 to 1.0 percent manganese, and up to 0. l0 percent silicon.

6. A method in accordance with claim 4, wherein the zirconium content of the steel is limited to a maximum of 0.07

percent.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,634 O73 Dated Qa a 1 912 Invent John G. Cutton, et 31 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 32, after "U.S.", insert Reissue Column 6, line 63, change "of" to read or Column 7, line 26, change "out" to read our Signed and sealed this 31st day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) usgg -pc eognmpgg x: u.s. covsnumsm' PRiNTING OFFICE: was o-2ss-ss|. 

2. A steel in accordance with claim 1, wherein the maximum zirconium content is 0.05 percent.
 3. A steel in accordance with claim 2, wherein the steel contains up to about 0.20 percent chromium.
 4. A method of producing free-machining resulfurized and rephosphorized carbon steel articles of improved chemical and microstructural homogeneity and enhanced hot workability, comprising partially killing a molten steel containing carbon in an amount from 0.30 weight percent to 0.45 weight percent, sulfur in an amount from 0.08 weight percent to about 0.15 weight percent and phosphorus in an amount from 0.07 weight percent to about 0.12 weight percent, and adding thereto a zirconium-containing material in amount sufficient to provide the steel with a zirconium content of at least about 0.015 weight percent.
 5. A method in accordance with claim 4, wherein the steel contains from 0.70 to 1.0 percent manganese, and up to 0.10 percent silicon.
 6. A method in accordance with claim 4, wherein the zirconium content of the steel is limited to a maximum of 0.07 percent. 